In the design for the retryable reads we decided that drivers should also support retryable reads against 3.6 and 4.0 server in addition to 4.2. However, the driver test suite for retryable reads heavily relies on the failCommand fail point, which doesn't exist in 3.6.

If we don't backport the fail point then retryable reads will be largely untested against a 3.6 server but I don't think this is necessarily a blocker. Retrying is a driver feature and does not depend on server behavior so a driver that works on 4.0 and 4.2 should also work on 3.6. Additionally, we have another driver project that will test retryable reads/writes and transactions against real failover scenarios which should add the lacking test coverage on 3.6.